The Ability Of Alpha Radiation To Penetrate Human Skin
By Miles Goldstick
September 1992
Adapted from: Golstick, Miles. April 1991. “The Hex Connection, Some Problems And Hazards
Associated With The Transportation Of Uranium Hexafluoride.” 196 pp. ISBN: 91-576-4440-3.
Ph.D. dissertation. Swedish University of Agricultural Sciences, Department of Ecology and
Environmental Research, Box 7072, S-750 07 Uppsala, Sweden. See pp. 82-88.
Compared to other forms of
radiation, alpha radiation has a
relatively low penetrating power.
Authors for or against nuclear
power, worried or not worried
about the effects of radiation,
use the same type of graphics
illustrating the penetrating ability
of radiation. In these graphics,
alpha radiation is illustrated as
being stopped by the skin or
a piece of paper. An example
of such a graphic is included
below. It was published in 1982
by the IAEA in a booklet titled
“Radiation Is A Fact Of Life.” [1]
The main perpetrator of
the belief that alpha radiation
cannot penetrate the skin is the
International Commission For
Radiological Protection (ICRP). It
is ICRP recommendations, based on the belief that
alpha radiation cannot penetrate the skin, that are
the basis for many official claims that alpha radiation
presents a low radiological hazard. For example,
the U.S. Department of Energy (DOE) wrote in 1987
that,
“Alpha particles resulting from the primary
disintegration of uranium present no external
radiation problem, since they do not penetrate
the skin.” [2]
Following is an examination
of the validity of this belief.
However, it is not contested
that radionuclides pose the
greatest threat to human
health when they are inhaled
or ingested. With regards
to alpha radiation, the short
distance alpha particles are
capable of travelling and their
low penetrating power make
it especially important to
distinguish between internal
and external exposure. This
is particularly important where
radiation exposure to workers
determines the time limit the
workers are allowed to carry
out certain tasks.
In the event of external
exposure to alpha radiation
the skin will receive a radiation dose. To determine
if alpha radiation can penetrate human skin deep
enough to reach living cells it is necessary to
define thickness of the outermost skin layer (called
epidermis) and determine the significance of sweat
pores. These problems are discussed below,
followed by results of a skin cancer frequency study
among Czechoslovakian uranium miners.
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Defining Epidermal Thickness,
The Study By Judi Whitton
In the early 1970s Judi Whitton recognized the
significance of epidermal thickness for radiological
protection. Sponsored by the Central Electricity
Generating Board (CEGB, a then British government
owned nuclear utility company), she carried out a
detailed literature survey to try and determine the
origin and accuracy of epidermal thickness values
then in use. Whitton wrote that “ICRP Publication 2”
(1959, p. 9) refers to a meeting of the Permissible
Dose Conference at Chalk River, Canada in
September 1949 where a value of 77 microns was
chosen for minimum epidermal thickness. The
source of this measurement is given as a histology
[3] textbook from 1942, which in turn gives no
original source of the data. [4]
A possible source of the ICRP 77 micron value
was traced by Whitton from a presentation made
at a 1963 symposium held in England called
“Radiation and Health.” This presentation also gave
a value of 77 microns for epidermal thickness,
and listed the source as two German studies from
1876 and 1879. Since some of the results of the
1879 study correspond to those given in the 1942
histology textbook, it is possible that the ICRP value
originates there. Whatever the purpose of the 1879
experiment, it could not have been performed for the
purpose of setting radiation protection standards, as
the modern nuclear era did not come into being until
several decades later. Further, formation of what
was to become the ICRP did not happen until 1928,
about a half-century later.
A fundamental aspect of any epidermal
thickness experiment is whether or not it takes into
consideration the natural elasticity of the skin. When
epidermis is removed from a body it naturally shrinks
and curls up. Results of an epidermal thickness
experiment are therefore much more accurate when
correction is made for shrinkage of the epidermis.
Comparison of results from experiments that did
and did not take into consideration shrinkage of
the epidermis shows that the value for epidermal
thickness is about half when shrinkage is taken into
account. [5] Regardless of the origin of the ICRP
value of 77 microns, the number is clearly within the
range of results from experiments that did not take
into consideration shrinkage of the epidermis.
As Whitton’s literature survey was unable to
confirm the origin and accuracy of epidermal
thickness values then in use, a project was set up
to make original measurements that overcome the
The Ability Of Alpha Radiation To Penetrate Human Skin
inadequacies of previous experiments. The problem
of shrinkage of the epidermis was overcome by
marking a circular area of epidermis to cut out,
tracing the size of this area onto a template, cutting
the epidermis around the mark, then stretching and
pinning it down onto the template corresponding to
the original size. The normal measuring procedure
can then be carried out. Since large samples of
epidermis are required for this technique, the
thickness measurements derived from epidermis
from a dead body were converted to weight
measurements of a small sample. Sampling was
then carried out on live volunteers using a three
millimeter diameter punch. Two weight/thickness
correlation factors were used; one for thick
epidermal areas such as the palm of the hand and
soles of the feet (100 micrograms is approximately
equal to 33.3 microns) and the second for other
epidermal areas such as the face and trunk (100
micrograms is approximately equal to 24.4 microns).
Whitton concluded:
“Epidermal thickness has been shown to
be in general less than is usually assumed
at present. In particular the value of 7 mg/
cm2 (77 microns) [6] suggested by ICRP for
‘minimal epidermal depth’ appears to be an
over-estimate for most body sites.
“As health physicists have assumed in the
past that minimal epidermal depth is 7 mg/cm2
(77 microns), skin dose from alpha emitters
external to the body has been ignored. From
the present study it is clear however that on
many parts of the body the basal layer [7] lies
within the range of several high energy alpha
emitters.” [8]
This conclusion is supported by observations
that alpha sources can cause abnormal redness of
the skin, [9] and by studies of skin cancer frequency
among Czechoslovakian uranium miners (discussed
below).
Whitton found that epidermal thickness can vary
over the body from a mean of 44 to 440 microns.
[10] However, the ICRP uses a uniform value for the
whole body, despite it being common knowledge
that epidermal thickness varies greatly between
different parts of the body. For general skin dose
control, Whitton recommended replacing the value
of 77 microns for minimal epidermal thickness with
a value of 44 microns for mean epidermal thickness.
[11] In 1977, the ICRP recommended use of a
mean epidermal thickness value of 70 microns, and
noted epidermal thickness may range from 50 to
100 microns. [12] This ICRP recommendation was
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made four years after publication of the research by
Whitton reviewed here.
Providing Whitton’s measurements are correct,
several alpha emitting isotopes, including U238
and U235, have a particle energy great enough to
penetrate the epidermis where it is thinnest on the
body. Some examples of the approximate mean
depth different alpha particles can penetrate tissue
are: americium-241, 54 microns; U235, 43 microns;
and U238, 41 microns. [13] Though these U235
and U238 values are below the mean minimum
epidermal thickness of 44 microns noted above, it
must be remembered that 44 microns is the mean
minimum not the absolute minimum. In some cases
epidermal thickness is greater and in some cases
less than 44 microns.
Whitton further concluded:
“No obvious correlation of epidermal thickness
with sex was observed on any of the body
sites. Similarly there appears to be no
consistent change in epidermal thickness with
increasing age on any body site.” [14]
Sweat Pores
One may assume from the Whitton study that
parts of the body where the epidermis is thickest,
such as the palm of the hand and finger tips, that
the epidermis is sufficiently thick to block the most
energetic alpha particles. This is not the case.
Whitton fails to mention that for fragments as tiny
as alpha emitters the existence of eccrine sweat
glands, commonly called sweat pores, and hair
follicles all over the skin present a barrier with few
but large holes. What is more, the palm of the hand,
though having extra thick epidermis, has a greater
density of sweat pores than areas of the body where
the epidermis is thinner.
The average density of sweat pores varies
greatly with the individual and body site. The total
number of sweat pores distributed over the entire
body has been estimated at from 1.6 to four million;
and the number on specific body sites as 64/cm2 on
the back, 108/cm2 on the forearm, 181/cm2 on the
forehead, and 600 to 700/cm2 on the palms of the
hand and soles of the feet. [15]
The size of the sweat gland has been found to
vary as much as fivefold between individuals. The
dimension of the coil leading down from the opening
in the epidermis is about two to five mm long and
about 60 to 80 microns in diameter, with the duct
having a slightly smaller diameter. [16] In contrast,
the diameter of a uranium oxide particle is about one
micron. [17] The diameter of an alpha emitter itself
The Ability Of Alpha Radiation To Penetrate Human Skin
is more than one thousand times smaller than the
diameter of a sweat pore. The spiraling nature of the
sweat gland coil leading down from the opening may
limit the depth into the epidermis that alpha particles
and emitters reach. Further, the percentage of total
surface area of epidermis made up of sweat gland
openings is small, being only a few percent in areas
where sweat pores are most dense. Nevertheless,
alpha emitters can cause skin cancer after fastening
in sweat pores despite:
- the small percentage of total surface area of the
epidermis made up of sweat gland openings,
- the coiling nature of sweat glands,
- and other considerations such as sweat gland
openings being plugged, and the angle of entry.
In addition, alpha emitters can stick to any part of
the skin.
Skin Cancer Among Czechoslovakian
Uranium Miners
Analysis has been done of skin cancer frequency
in a group of several thousand Czechoslovakian
uranium miners over an eight year period (19681975). The study concluded that the incidence of
skin cancer (basal cell carcinoma) was significantly
higher than expected (by almost 10 times),
especially among miners who had worked for 10
years or more. The predominant location of facial
cancers was the cheek and forehead, places where
skin is especially thin. [18]
Dose estimates were made from radon daughters
to the skin. The dose from uranium and other alpha
emitters was not included. It was found that after
working 10 years or more, miners could receive
a cumulative dose equivalent of 10-20 Sv. The
study concluded that such high doses to the basal
epidermal layer (where live cells nearest to the
surface of the skin are located) is evidence that
external alpha radiation from radon daughters
may be the cause of the cancers. [19] The authors
concluded,
“The results of this short-term observation
indicate the actual possibility of a carcinogenic
effect of external alpha irradiation of the skin.” [20]
Adding hope to their dismal conclusion, the
authors noted,
“The consequences of skin cancer are
much less severe than the consequences of
malignant tumors in other organs because skin
cancer occurs in accessible places, allowing
an early diagnosis and more successful
surgical therapy.” [21]
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Conclusion
Contrary to popular belief, alpha radiation is not
always blocked by the outermost skin layer (the
epidermis) and external exposure to alpha radiation
may cause cancer. Several alpha emitting isotopes,
including U238 and U235, have a particle energy
great enough to penetrate the epidermis where it
is thinnest on the body. In addition, alpha emitters
can fasten in hair follicles and sweat pores and stick
to any part of the skin. For these reasons, external
dose to the skin from alpha emitters must be taken
into account in determining the health effects of
alpha radiation.
Notes
[1] IAEA, June 1982, p. 3.
[2] Oak Ridge Operations, September 1987, p. 56.
[3] Histology is the branch of biology dealing with the
study of tissues.
[4] Whitton, 1972, p. 1; in: Health Physics, Vol. 24,
January 1973.
[5] Whitton, 1972, pp. 4-5; in: Health Physics, Vol. 24,
January 1973.
[6] One mg/cm2 is approximately equal to 11 microns for
tissue, according to Whitton, p. 1; in: Health Physics, Vol.
24, January 1973.
[7] The “basal layer” contains the live cells closest to the
surface of the skin.
[8] Whitton, 1972, pp. 7-8; in: Health Physics, Vol. 24,
January 1973.
[9] Whitton, 1972, p. 7; in: Health Physics, Vol. 24,
January 1973.
[10] Whitton, 1972, p 1; in: Health Physics, Vol. 24,
January 1973.
[11] Whitton, 1972, p. 8; in: Health Physics, Vol. 24,
January 1973.
[12] International Commission For Radiological Protection
(ICRP), March 1987, p. 13.
[13] These micron values are based on a formula
presented in: Sorenson, James A. and Phelps, Michael
E., 1980, p. 148. The average alpha particle energies are
listed in: Browne, Edgardo and Firestone, Richard B.;
Shirley Virginia S. (ed), 1986.
[14] Whitton, 1972, p. 5; in: Health Physics, Vol. 24,
January 1973.
[15] Sato, et. al., 1989, p. 538; in: Journal Of The
American Academy Of Dermatology,
Vol. 20, No. 4. April 1989.
[16] Sato, et. al., 1989, p. 539; in: Journal Of The
American Academy Of Dermatology,
Vol. 20, No. 4. April 1989.
[17] The size of uranium oxide particles is taken from a
22 January 1981 report by Mark Goldberg prepared for
the Bjarnie Paulson court case on behalf of the Canadian
Coalition For Nuclear Responsibility, Box 236 Snowdon
Post Office, Montreal, Quebec, Canada. H3X 3T4. Tel.
1-514-489-2665.
[18] Sevcova, et. al., 1975, p. 805; in: Health Physics,
Vol. 35, December 1978.
[19] Sevcova, et. al., 1975, p. 805; in: Health Physics,
Vol. 35, December 1978.
[20] Sevcova, et. al., 1975, p. 806; in: Health Physics,
Vol. 35, December 1978.
[21] Sevcova, et. al., 1975, p. 806; in: Health Physics,
Vol. 35, December 1978.
Bibliography
International Atomic Energy Agency (IAEA). June
1982. “Radiation - A Fact Of Life.” 16 pp. IAEA,
Box 100, A-1400 Vienna, Austria.
International Commission For Radiological
Protection (ICRP). March 1987. “ICRP
Publication 26, Recommendations Of The ICRP,
2nd Edition.” 84 pp. Pergamon Press. ISBN: 008-0215114.
Oak Ridge Operations. September 1987. “Uranium
Hexafluoride: Handling Procedures And
Container Descriptions.” 79 pp. ORO-651 (Rev.
5) (DE87014088). U.S. DOE, Box E, Oak Ridge,
Tennessee, U.S.A. 37831.
Sato, K; Kang, W. H.; Saga, K; and Sato, K. T.
April 1989. “Biology Of Sweat Glands And
The Ability Of Alpha Radiation To Penetrate Human Skin
Their Disorders. I. Normal Sweat Gland
Function.” Journal Of The American Academy Of
Dermatology, Vol. 20, No. 4. April 1989, pp. 537xxx.
Sevcova, M.; Sevc, J.; and Thomas, J. 1975. “Alpha
Irradiation Of The Skin And The Possibility Of
Late Effects.” 4 pp. Health Physics, Vol. 35,
December 1978, pp. 803-806.
Whitton, Judi T.; Central Electricity Generating
Board, Berkeley Nuclear Laboratories, Berkeley,
Glos. 14 February 1972. “New Values For
Epidermal Thickness And Their Importance.” 8
pp. Health Physics, Vol. 24, January 1973, pp.
1-8. Pergamon Press.
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